1. Field of the Invention
The present invention relates to a method of forming a trench isolation structure on a silicon substrate, where the trench is lined with silicon nitride.
2. Description of the Related Art
Shallow trench isolation structures are used to isolate circuit elements on an LSI chip. The sidewalls of the trenches are lined with silicon nitride to relief the stress caused by oxidation before the trenches are filled with a silicon dioxide filler. FIGS. 1A to 1G illustrate a number of steps currently used to form a shallow trench isolation structure. On a silicon substrate 1 a pad oxide layer 2 and a pad nitride layer 4 are sequentially deposited and through an aperture of resist 6 the layers 2 and 4 are anisotropically etched (FIG. 1A). After removing the resist 6, the pad nitride layer 4 is used as a mask for etching the silicon substrate 1 to form a trench 8 (FIG. 1B). In a thermal oxidation step, the inside of the trench is coated with a thermal oxide liner 10 to relief the damage produced during the trench formation. The wafer is then coated with a silicon nitride liner 12 (FIG. 1C). Silicon dioxide is then deposited on the wafer to fill the trench 8 with a filler 14 (FIG. 1D). Silicon dioxide filler 14 is densified by subjecting the wafer to an annealing process and the wafer is planarized by the CMP or etchback method until the silicon nitride liner 12 or pad nitride layer 4 is exposed (FIG. 1E). Hot phosphoric acid is used to remove the pad nitride layer 4 (FIG. 1F). Finally, hydrofluoric acid is used to remove the pad oxide layer 2 and a portion of the filler 14 that lies above the surface of the substrate 1 (FIG. 1G).
However, when the pad nitride layer 4 is removed by hot phosphoric acid, the acid penetrates down along the silicon nitride liner 12 and produces an unacceptable recess 16 as shown in FIG. 1F. As a result, when the surplus portion of the filler 14 is removed, the hydrofluoric acid penetrates through the recess 16 and produces a recess 18 in the filler 14 as shown in FIG. 1G.
In order to solve these problems, Fahey et al. U.S. Pat. No. 5,447,884 discloses that the silicon nitride liner 12 has a thickness less than 5 nm. However, Benedict et al. U.S. Pat. No. 5,763,315 discloses that if the silicon nitride liner 12 is deposited equal to or less than 4 nm in thickness, the liner is not an effective O2 diffusion barrier and defects are readily formed in the trench capacitor array.
It is therefore an object of the present invention to provide an improved method of forming a trench isolation structure resistant to hot phosphoric acid without restrictions on the thickness of silicon nitride liner.
The stated object is obtained by introducing lateral component to the hot phosphoric acid penetration on the shoulder portions of the trench structure so that when the stripping process is complete the head of the lateral penetration stops short of the upper edges of the silicon nitride liner.
According to a first aspect of the present invention, there is provided a method of forming a trench isolation structure in a silicon substrate, comprising the steps of depositing a masking layer of nitride having an aperture on the substrate, forming a spacer of oxide on inner sidewall of the aperture to define a mask window, etching the substrate through the mask window to form a trench, removing the spacer to form stepped shoulder portions on upper edges of the trench, thermally depositing a liner of oxide in the trench, depositing a liner of nitride on an area including the trench and the stepped shoulder portions, depositing an isolation filler of oxide in the trench, and etching the layer of nitride with hot phosphoric acid.
According to a second aspect, the present invention provides a method of forming a trench isolation structure in a silicon substrate, comprising the steps of depositing a layer of nitride on the substrate and a masking layer of oxide on the layer of nitride so that the layers have a conformal aperture, forming a spacer of oxide on inner sidewall of the conformal aperture to define a mask window, etching the substrate trough the mask window to form a trench, removing the spacer and the masking layer to form stepped shoulder portions on upper edges of the trench, thermally depositing a liner of oxide in the trench, depositing a liner of nitride on an area including the trench and the stepped shoulder portions, depositing an isolation filler of oxide in the trench, and etching the layer of nitride with hot phosphoric acid.